The primary objective of this work is to elucidate the cellular and synaptic mechanisms responsible for the genesis of rhythmic activity by the vertebrate spinal cord. Experiments are performed on isolated preparations of the chick and mouse spinal cords maintained in vitro. Most of our effort is focused on understanding the mechanisms responsible for locomotor activity by the mouse spinal cord. We have discovered that electrical activation of motoneurons ? generally considered the output elements of the spinal cord ? can trigger an episode of locomotor-like activity. This suggests the existence of unexpected connections between motoneurons and the spinal circuitry responsible for generating locomotion. We are investigating these connections using optical and electrophysiological methods. We have also developed a new method based on electroporation to load calcium-sensitive dyes into neurons. We are applying this technique to the mouse spinal cord in combination with 2-photon microscopy to identify neurons activated by motoneuron stimulation. We have found evidence that ventral root stimulation results in an unexpected monosynaptic glutamatergic response in Renshaw cells and possibly other motoneurons. We are now investigating these connections to understand their role in locomotion.